Doped Gallium Nitride Annealing

ABSTRACT

The present invention involves annealing methods for doped gallium nitride (GaN). In one embodiment, one method includes placing, within a heating unit, a silicon carbide (SiC) wafer as a susceptor in close proximity with a doped GaN epilayer, wherein the doped GaN epilayer is either a GaN layer grown on a substrate or a GaN layer that is free standing; and heating, at a heating rate of at least about 100° C./s, the wafer and the doped GaN epilayer to at least about 1200° C. In another embodiment, another method includes placing, within a heating unit, a doped GaN epilayer, wherein the doped GaN epilayer is either a GaN layer grown on a conducting substrate or a GaN layer that is free standing; and heating, at a heating rate of at least about 100° C./s, the doped GaN epilayer to at least about 1200° C.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit of provisional patentapplication Ser. No. 61/051,902 to Sundaresan et al., filed on May 9,2008, entitled “Nanowire Growth Using Microwave Heating-AssistedPhysical Vapor Transport,” which is hereby incorporated by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under grantW911NF-04-1-0428 awarded by the Army Research Office; SBIR grant no.0539321 awarded by National Science Foundation (NSF); grant no. DMR05-20471 awarded by NSF UMD MRSEC; and award nos. ECS-0618948 andECCS-0742139 both awarded by NSF. The government has certain rights inthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a flow diagram of an annealing method for doped GaN as oneaspect of the present invention.

FIG. 2 shows a flow diagram of an annealing method for doped GaN asanother aspect of the present invention.

FIG. 3 shows an example of a block diagram of a solid-state microwaveannealing system.

FIG. 4 shows another example of a block diagram of a solid-statemicrowave annealing system.

FIG. 5 shows examples of XPS survey scans of: (a) AlN as-capped GaNsample, (b) AlN capped sample after 1400° C./5 s annealing, and (c)after removal of the AlN cap at the conclusion of annealing.

FIG. 6 shows an example of low-temperature (5 K) PL spectra of bothas-grown Mg-doped GaN and AlN capped samples subjected to 5 s microwaveannealing at 1300° C. and 1500° C.

FIG. 7 shows an example of low-temperature (5 K) PL spectra of as-grownMg-doped GaN and e-beam deposited MgO capped in-situ Mg-doped GaNsamples after 5 s microwave annealing at 1300° C. and 1350° C.

FIG. 8 shows an example of hole concentration as a function of annealingtemperature for 5 s duration microwave annealing on uncapped, MgOcapped, and AlN capped in-situ Mg-doped GaN.

FIG. 9 shows an example of a comparison between simulated andexperimental (as-implanted) Mg multiple energy implant profile in GaN.

FIG. 10 shows examples of SIMS depth profiles of the Mg implanted GaNbefore and after 1300° C./5 s and 1400° C./5 s microwave annealing.

FIG. 11 shows an example of low-temperature (5 K) PL spectra from anun-implanted GaN epilayer, and GaN epilayers before and after 1400° C./5s and 1500° C./5 s microwave annealing.

FIG. 12 shows an example of XRD spectra of the multiple-energy Mgion-implanted and the microwave annealed samples for 5 s duration.

FIG. 13 shows an example of XRD scans of the GaN (004) for single energyMg as-implanted and microwave annealed (15 s duration) films.

FIG. 14 shows an example of low-temperature (5 K) PL spectra from anun-implanted GaN epilayer, and multiple energy Mg-implanted GaNepilayers before and after 1400° C./5 s and 1500° C./5 s microwaveannealing.

FIG. 15 shows an example of low-temperature (5 K) PL spectra ofsingle-energy Mg:GaN film grown on 4H—SiC; and of AlN-capped samplessubjected to 15 s microwave annealing at 1300° C., 1400° C., and 1500°C.

FIG. 16 shows an example of two-probe I-V measurements on the singleenergy Mg-implanted, 15 s microwave annealed samples.

DETAILED DESCRIPTION OF THE INVENTION

The present invention embodies novel heating techniques for dopantannealing. In one embodiment, discloses annealing methods for dopedgallium nitride (GaN). In one instance, the doped GaN is grown on anonconducting substrate. In another instance, the doped GaN is grown ona conducting SiC substrate. In yet another embodiment, dopant annealingmay be applied in semiconducting materials.

Referring to FIG. 1, a doped GaN annealing method is shown. As oneembodiment of this method comprises placing, within a heating unit, aSiC wafer as a susceptor in close proximity with a doped GaN epilayer.The doped GaN epilayer may either be a GaN layer that is grown on asubstrate or a GaN layer that is free standing S105. The thickness ofthe GaN layer may be less than or equal to about 1 mm.

Close proximity means either one element (i.e., SiC wafer) touchinganother element (i.e., GaN epilayer) or at most a distance of about 500micrometers. Furthermore, the SiC wafer may be located above, below,near, or to the side of the doped GaN epilayer. The SiC may be ahexagonal SiC (e.g., 4H—SiC, 6H—SiC, etc.) or cubic SiC (e.g., 3H—SiC).The purpose of using SiC as a susceptor is because SiC acts as a goodabsorber of microwaves and consequently heats up rapidly. In otherwords, the SiC wafer as a conduit for the heating (annealing) of theepilayer.

The substrate used may either be a nonconducting substrate or aconducting substrate. Where a nonconducting substrate is used, thenonconducting substrate may be a semi-insulating substrate (e.g., SiC,etc.) or an insulating substrate (e.g., sapphire, etc.). Where aconducting substrate is used, the conducting substrate may be a dopedSiC substrate, a conducting GaN substrate, or any other conductingsubstrate. After such placement, the wafer and the epilayer are heatedto at least about 1200° C. S110. The rate of heating is at least about100° C./s.

The doped GaN may either be in situ doped or ion-implantation doped. Thedopant used for the doped GaN can be magnesium, beryllium, calcium,zinc, silicon, sulfur, iron, cobalt, vanadium, or any combination ofthese elements.

The method may further include depositing a protective capping layer onthe GaN epilayer. In one embodiment, the protective capping layer isaluminum nitride (AlN). AlN may be deposited using pulsed-laserdeposition.

In another embodiment, the protective capping layer is magnesium oxide(MgO). The MgO may be deposited using electron beam evaporation of anMgO target. The MgO target may be fused lumps of MgO, wherein the fusedlumps comprise of a multitude of about 3 to about 12 mm pieces of 99.95%metals basics.

The method may be performed in a vacuum chamber or an inert gasatmosphere (e.g., helium, neon, argon, krypton, or xenon).

The method can use any kind of known heating unit. For example, theheating unit may be a microwave heating head (such as the Microwave RTPSystem by LT Technologies of Fairfax, Va.). In another example, theheating unit may be a laser annealing system (such as LA-TF or LaserAnneal 6300, both by AMBP Tech of Piscataway, N.J.). In yet anotherexample, the heating unit may be a halogen or mercury lamp.

While these heating units are known, the present invention's doped GaNannealing techniques remain novel. It is within the scope of the presentinvention that such annealing techniques include, but are not limitedto, microwave heating annealing, laser annealing, halogen lampannealing, mercury lamp annealing, etc. Although the followingembodiments use microwave heating annealing as an example, any of theseexemplified techniques may be used to achieve the requisite high heatingrates and temperatures of the present invention.

Referring to FIG. 2, another doped GaN annealing method is shown.However, the method here does not involve a susceptor or a nonconductingsubstrate. Rather, the method comprises placing, within a heating unit,a doped GaN epilayer S205. The doped GaN epilayer may either be a GaNlayer that is grown on a substrate or a GaN layer that is free standingS105. The substrate used may be a conducting, insulating, orsemi-insulating GaN substrate. The thickness of the GaN layer may beless than or equal to about 1 mm. Afterwards, the epilayer is heated toat least about 1200° C. S210. The rate of heating is at least about 100°C./s.

The doped GaN may either be in situ doped or ion-implantation doped. Thedopant used for doping GaN can be magnesium, beryllium, calcium, zinc,silicon, sulfur, iron, cobalt, vanadium, or any combination of theseelements.

Furthermore, this method may also include depositing a protectivecapping layer on the GaN epilayer. Like above, as one embodiment, theprotective capping layer is AlN. AlN may be deposited using pulsed-laserdeposition. In another embodiment, the protective capping layer is MgO,where the MgO may be deposited using electron beam evaporation of an MgOtarget. The MgO target may be fused lumps of MgO, wherein the fusedlumps comprise of a multitude of about 3 to about 12 mm pieces of 99.95%metals basics. Moreover, the method may be performed in a vacuum chamberor an inert gas atmosphere (e.g., helium, neon, argon, krypton, orxenon).

Similarly, this method can use any kind of known heating unit. Forexample, the heating unit may be a microwave heating head (such as theMicrowave RTP System by LT Technology of Fairfax, Va.). In anotherexample, the heating unit may be a laser annealing system (such as LA-TFor Laser Anneal 6300, both by AMBP Tech of Piscataway, N.J.). In yetanother example, the heating unit may be a halogen or xenon lamp.

While these heating units are known, the present invention's doped GaNannealing techniques remain novel. It is within the scope of the presentinvention that such annealing techniques include, but is not limited to,microwave heating annealing, laser annealing, halogen lamp annealing,mercury lamp annealing, etc. Although the following embodiments usemicrowave heating annealing as an example, any of these exemplifiedtechniques may be used to achieve the requisite high heating rates andtemperatures of the present invention.

I. Introduction

GaN belongs to a class of semiconductors known as wide band-gapsemiconductors. GaN is important for high-power solid-state devices,especially for those intended for microwave frequency range and also foroptoelectronics applications on account of its direct band gap. The GaNbased high electron mobility transistors (HEMTs) have definedstate-of-the-art for output power density and have the potential toreplace GaAs based transistors for a number of high-power applications.The advantages of GaN over other semiconductors include: a highbreakdown field (3 MV/cm, which is ten times larger than that of GaAs),a high saturation electron velocity (2.5×107 cm/s), and the capacity ofthe III-nitride material system to support heterostructure devicetechnology with a high two-dimensional electron gas (popularly known as2DEG) density and high carrier mobility.

Another attractive feature of all III-nitride semiconductors is thepossible polarization-induced bulk three-dimensional doping withoutphysically introducing shallow donors. The strong piezoelectric effectand a large spontaneous polarization in the III-nitride system allowsfor the incorporation of a large electric field (>106 V/cm) and a highsheet charge density (>1013 cm-2) without doping. This effect andpolarization help to realize a variety of high-performance andhigh-power microwave devices.

As indicated in the following table, TABLE 1 shows a comparison ofmaterial properties of several semiconductors, namely Si, GaAs, SiC, andGaN.

TABLE 1 Material Properties of Semiconductors Si, GaAs, and GaNAttribute Si GaAs GaN Energy Gap (eV) 1.11 1.43 3.4 Breakdown E-Field(V/cm) 6.0 × 10⁵ 6.5 × 10⁵ 3.5 × 10⁶ Saturation Velocity (cm/s) 1.0 ×10⁷ 2.0 × 10⁷ 2.5 × 10⁷ Electron Mobility 1350 6000 1600*    (cm²/V-s)Thermal Conductivity 1.5 0.46 1.7 (W/cmK) Heterostructures SiGe/SiAlGaAs/GaAs AlGaN/GaN InGaP/GaAs InGaN/GaN AlGaAs/InGaAs *Typicaltwo-dimensional electron gas mobility for AlGaN/GaN heterostructures.

Ion-implantation is an indispensable technique for selective area dopingof GaN, for fabricating high-power electronic and opto-electronicdevices. Other doping methods, such as thermal diffusion, areimpractical for GaN technologies because the diffusion co-effecients ofthe technologically relevant dopants in GaN is very small, even attemperatures in excess of 1800° C.

However, being a highly energetic process, ion-implantation can damagethe semiconductor crystal lattice. Moreover, the as-implanted dopantstend not to reside in electrically active substitutional sites in thesemiconductor lattice. Therefore, ion-implantation always needs to befollowed by a high-temperature annealing step for alleviating theimplantation-induced lattice damage and for activating the implanteddopants (i.e., moving them from interstitial to electrically activesubstitutional lattice sites).

For implanted n-type dopants (e.g., Si) in GaN, annealing temperaturesin the range of 1200° C. are required, whereas implanted p-type dopants(e.g., Mg and Be) in GaN require annealing temperatures in excess of1300° C. for (1) satisfactorily removing implantation-induced latticedamage, (2) activating the implanted dopants, and (3) recovering theluminescence properties (which are severely degraded by theion-implantation). The higher temperature requirement for activatingp-type implants compared to n-type implants in GaN is primarily due tothe much larger formation energy of the substitutional Mg_(Ga) speciescompared to the Si_(Ga) species.

Temperatures over 1300° C. are required for completely activatingin-situ, as well as ion-implanted p-type dopants. However, when annealedat temperatures above 800° C., GaN decomposes into Ga droplets due tothe nitrogen leaving the surface. Annealing of GaN can be performed inhalogen lamp-based RTA systems because of the rapid heating/coolingrates accorded by these RTA systems. However, due to their quartzhardware, these halogen lamp based RTA systems are limited to a maximumtemperature of 1200° C., which is not sufficient to effectively annealp-type GaN.

A way to overcome these problems is using SSM heating. SSM heating isadvantageous for high-temperature processing of wide-bandgapsemiconductors, such GaN. This heating process has a capability to reachsample temperatures >2000° C. (for SiC wafers) with heating and coolingrates in excess of 600° C./s.

An example of a SSM RTP system 301 used in this work is illustrated inFIG. 3 and FIG. 4. This SSM RTP system has three main building blocks:(1) a variable frequency microwave solid state power source 305, whichconsists of a signal generator 310 and a power amplifier 315, (2) amicrowave heating system 320, which consists of a coupling and tuningcircuit 325 and at least one microwave heating head 330, 415, forcoupling microwave power to the targeted source wafer having a T1 405and (3) a measurement and control system 335, which consists of anetwork analyzer 340, a computer 345, an optical pyrometer 350, andother equipment. Below the targeted source wafer having a T1 405 andseparated by a small gap is the substrate wafer having a T2 410. Thetemperature difference between T1 and T2 forms the temperature gradientΔT. Microwave power generated by the variable frequency power source 305is amplified and then coupled to a SiC sample 355, 410 through themicrowave heating head 330. The environmental gas and pressure of thechamber can be controlled by vacuum pump 425 and external vapor/gassource 420. The sample temperature can be monitored through a viewport430 by an infrared pyrometer or the optical pyrometer 350. The SiC andGaN sample emissivities can be measured using a blackbody source. Themeasured emissivity value (e.g., 0.8) is then keyed into the pyrometerfor all temperature measurements.

In the case of annealing doped GaN, either 405 or 410 can represent thedoped GaN sample. The other one that is not represented by GaN may be aSiC susceptor. The susceptor may or may not be required.

The microwave system 415 above can be tuned to efficiently heatsemiconductor samples at variable frequencies. For instance, operatingat about 150 W, the frequency may range from about 910 MHz to about 930MHz. Temperature may be maintained at a steady state, such as ˜1800° C.,for a certain amount of time, such as about 15 seconds.

Since samples should be placed in an enclosure made of microwavetransparent, high-temperature stable ceramics (such as boron nitride andmullite), microwaves only heat the strong microwave absorbing(electrically conductive) semiconducting films, which present a very lowthermal mass in comparison with a conventional furnace where thesurroundings of the sample are also heated. Thus, heating rates >600°C./s are possible.

Microwave heating has an advantage of selective heating. When microwavepower radiates on two different materials, such as a highly doped SiCwafer (which tends to be a strong microwave absorber), and asemi-insulating SiC wafer (which tens to be a poor microwave absorber),microwaves may selectively heat up highly doped SiC of the strongmicrowave absorber while leaving a negligible direct, heating effect onthe semi-insulating SiC.

Selective heating of thin, highly doped semiconductor layers is possibleif the doped layers are formed on semi-insulating or insulatingsubstrates. Therefore, for efficient microwave annealing of implantedsemi-insulating (SI) SiC substrates and GaN epilayers grown on(electrically insulating) sapphire substrates, a 5 mm×5 mm heavily doped4H—SiC sample can be placed as a susceptor directly underneath thesample to be annealed. It is possible to directly couple microwave powerand heat GaN epilayers grown on sapphire, without using any susceptor.However, the spatial distribution of temperature across the sample maybe extremely non-uniform. Placing a SiC susceptor sample underneath theGaN sample of interest can solve this problem.

III. Microwave Annealing of In-Situ and Ion-Implanted Acceptor Doped GaN

A. Problems

GaN is a very important (direct) wide bandgap semiconductor forfabricating opto-electronic devices in the short-wavelength region andfor high-power/frequency devices. Reliable, planar, and selective areaacceptor doping technology is required for making high performance GaNdevices. Ion-implantation is the only post growth doping methodavailable for this purpose in GaN, as thermal diffusion of the dopantsis not feasible in this material owing to low nitrogen dissociationtemperature (˜900° C.).

Like SiC, the inability to achieve high p-type conduction in GaN has sofar limited the commercialization of this otherwise promisingsemiconductor for many electronic and opto-electronic applications.Similar to the strong Si—C covalent bond in SiC, the large ionicity ofthe Ga—N bond gives GaN its unique properties and makes it a difficultmaterial to work with technologically. TABLE 2 lists the best R_(s)values and other electrical properties reported to-date for p-type GaN.

TABLE 2 List of the Electrical Characteristics of p-type GaN Availablefrom Literature Specie and doping Dose Depth RTA Sheet Res. MobilitySheet Carrier method cm⁻² μm temp/time Ω/□ cm²/Vs Conc. cm⁻² Be implant 1 × 10¹³  0.25 1100° C./30 s 8.4 × 10⁴ 4.4 5.75 × 10¹³ Be implant  5 ×10¹⁴ 40 KeV PLA + RTA 8.4 × 10⁴ 8.7 8.56 × 10¹² 1100° C./120 s Be/N = 12.5 × 10¹⁴ 0.5 1200° C./10 s 5.6 × 10⁴ 6.8  2.7 × 10¹² Co-implant Mg/N =2 1.5 × 10¹⁵ 0.3 1200° C./10 s 5.6 × 10³ 7.7  1.4 × 10¹⁴ Co-implant

The minimum achievable R_(s) for doping with magnesium (Mg) andberyllium (Be), the popularly used p-type dopants in GaN, is in therange 10⁴ Ω/□-10⁵ Ω/□. It should be noted that throughout the presentinvention, the unit Ω/□ means Ω/sq. Clearly, these values are too highto permit one to fabricate high-performance electronic andopto-electronic devices. The difficulty in achieving low sheetresistance for p-type GaN may be attributed to the presence of highdensities of donor-type point defects such as nitrogen vacancies(V_(N)), and their complexes with native defects and acceptor dopants,which have relatively low formation energies. These defects are known tohave a donor behavior in GaN, thus restricting the maximum p-typeconduction. The achievement of high p-type conductivity is even moredifficult in ion-implanted GaN layers because the implantation-induceddamage creates extra donor-type defects, which compensate the activatedholes. Moreover, the optical properties of GaN are greatly diminished byion-implantation resulting in a complete loss of luminescence even forlow doses. The strongest effect is the creation of non-radiativerecombination centers due to the implant-induced damage. The introduceddefects have mainly deep levels within the bandgap; therefore, theas-implanted GaN is electrically highly resistive. The damage must beannealed out to achieve optical and electrical activation of theimplanted dopants.

Post implantation high-temperature (>1200° C.) annealing is required forremoving the implantation generated defects as well as to activate theimplanted dopants. Conventional annealing of GaN cannot be done attemperatures >900° C. for more than a few seconds, due to a lowdissociation temperature (900° C.) of N. The low ramping rate ofconventional annealing furnace also subjects the GaN samples to amuch-prolonged unintentional heating, which may further introducedefects in the GaN films. Hence, to preserve the surface morphology andthe lattice quality, the required annealing temperature should bereached very fast and the duration of anneal should be limited to fewseconds.

The fundamental diffusion, recovery, and activation processes that occurin ion-implanted impurities in GaN as a function of annealingtemperature are generally known. Optical activation can be achieved inthe temperature range 1200° C.-1300° C. But defect complexes (whichcause an intense yellow band in the photoluminescence (PL) spectra)require still higher temperatures (in excess of 1300° C.) to break up.Acceptor-type activation in GaN is much more difficult to achievecompared to donor-type activation due to presence of unintentionalcompensating deep donors (e.g., nitrogen vacancies, V_(N), and itscomplexes), as well as the absence of shallow acceptors. Halogen-lampbased rapid thermal annealing (RTA) is a common method used to annealthe defects, to repair the lattice damage and to activate the dopants inGaN. The lowest R_(s) values, 5.6×10⁴ Ω/□ for Be-implanted GaN, and5.6×10³ Ω/□ for Mg implanted GaN in TABLE 2, were achieved using RTA atan annealing temperature of 1200° C. and a short anneal duration of 10s.

AFM micrograph of a Be⁺-implanted GaN sample annealed by halogen-lampRTA at 1100° C. for 2 min can be taken. The RMS roughness extracted fromthe AFM image may be 2 nm, despite only scanning a 1 μm×1 μm squarearea.

High-resolution x-ray diffraction spectra θ-2θ may also be recorded for(a) a MBE as-grown GaN sample, (b) a Be-implanted sample aftercombination of PLA and RTA, (c) a Be-implanted sample after RTA, (d) aBe-implanted sample after PLA, and (e) a Be-implanted sample withoutannealing. Compared to the MBE as-grown sample, the Be as-implantedsample generally has a broadening of the main (0002) GaN peak and theappearance of an additional peak at the low-angle side, which isconsistent with the expansion of the GaN lattice in the implantedregion. Even after the PLA treatment, the lower angle peak remains. Thispresence indicates that the shallow penetration depth of the 248 nmlaser only anneals the near surface region and leaves the deeper crystaldefects untouched. After RTA at 1100° C. following the PLA, the lowerangle peak has disappeared. However, the main peak is still much broadercompared to the as-grown sample, indicating that the RTA temperature isnot high enough to anneal out all the implant-induced defects. Someresidual strain still exists in the GaN layer after annealing. Thisresult emphasizes the need for a RTA technique with a higher temperaturecapability.

Key requirements for optimum annealing conditions for GaN appear toinclude both a high annealing temperature and a short annealing time. Asa rule of thumb, an implanted semiconductor should be annealed up to atemperature of ⅔ of its melting point for damage recovery and dopantactivation. In the case of GaN, this temperature is about 1650° C.However, such high temperatures are well beyond the capability of mostcommercial halogen lamp-based RTA equipment, which only have a modesttemperature capability of <1200° C. As such, an annealing method withthe capability of high processing temperature (≧1300° C.) is needed.Compared to SiC, an additional difficulty arises in case of GaN. GaNcannot withstand slow heating rates and long duration anneals attemperatures ≧1000° C. because of an incongruent sublimation of GaN,which decomposes into a N-rich gas and a gallium rich liquid at highertemperatures. Hence, to preserve the chemical integrity of GaN, whilesimultaneously reducing the density of compensating defects, therequired anneal temperature should be reached very fast and theannealing duration should be limited to a few seconds. Furthermore,compared to SiC, the anneal duration for GaN has to be shorter topreserve surface integrity. This requirement on ultra-fast heating ratesis not met by most conventional equipment.

There exists an RTP unit based on a MOCVD system has been built thatemploys RF heating with heating rates of 50° C./s. It supposedly annealsion-implanted GaN, capped with a layer of aluminum nitride (AlN) in thetemperature range 1200-1500° C. It has been reported that temperaturesas high as 1400° C. are required for alleviating the implantationinduced lattice damage and optimally activating the implanted dopants.

The sheet carrier concentration and mobility of Si-implanted GaNsubjected to annealing using such existing RTP unit may be comparedagainst each other. Generally, there is an improvement in the AlNencapsulated material quality up to 1400° C., but the results degradefor annealing temperatures >1400° C. This degradation is due toreliability issues associated with the AlN cap at higher temperatures,and possibly because the heating rates (50° C./s) are still not highenough to prevent GaN decomposition. However, with the much higherheating rates achievable with the microwave annealing system used inthis embodiment, there is the possibility of reliably annealing GaN attemperatures higher than 1400° C.

B. Microwave Annealing of In-Situ Mg Doped GaN

1. Experimental Details

The samples explored in this experiment were 3 μm thick Mg-doped GaNepilayers on a-plane sapphire substrates grown by metalorganic chemicalvapor deposition (MOCVD). To heat the GaN sample, a 5 mm×5 mm highlyconducting 4H—SiC piece is placed directly underneath the GaN sample ofinterest to serve as the susceptor, when both the GaN sample and the SiCpiece are placed within the microwave heating head. Microwave annealingof GaN is performed with and without a surface capping layer composed ofMgO, AlN or graphite. The AlN layers (200 nm thick) were deposited onthe GaN sample using pulsed-laser deposition. The MgO layers (200 nmthick) were deposited on the GaN using electron beam evaporation of aMgO target. Fused lumps of MgO (Alfa Aesar, 99.95%, metals basics, 3-12mm pieces) were used as target material. Graphite caps are formed on theGaN epilayers by first spin-coating a layer of standard photoresist,followed by annealing in vacuum at 750° C. Microwave annealing isperformed in the temperature range of 1300-1350° C. for short 5 sdurations in a pure (99.999%) nitrogen atmosphere. After microwaveannealing, the MgO cap is removed by etching in dilute acetic acid,whereas the AlN cap is removed by a 10 min etch in 85 wt% H₃PO₄ at 80°C.

The reliable application and removal of the AlN cap on the GaN surfacewas studied using XPS. The XPS spectra were acquired using a Mg Kα x-raysource. The sample surface after annealing and removal of the cap ismonitored by tapping mode AFM. The optical characterization of thematerial is performed using low-temperature (5 K) photoluminescence (PL)spectroscopy. For obtaining the PL spectra, a He—Cd laser was used withan excitation intensity of 2.5 mW. Details about the PL system aregenerally known. Room temperature Hall measurements were performed afterdepositing (30 nm) Ni/(30 nm) Au contacts on the GaN layers in the vander Pauw geometry. The contacts were made ohmic by alloying in aconventional box furnace at 350° C., in air, for 10 min.

2. XPS Characterization of AlN Capped GaN

The reliability of the application, sustainability of the AlN cap duringannealing, and removal of the AlN cap after microwave annealing wasstudied by XPS. Referring to FIG. 5, part (a) shows a survey XPS scan ofthe surface of the AlN as-capped sample. Other than O 1s and C 1ssignals coming from the native oxide/hydrocarbon layer, only Al and Nsignals can be seen in the survey scan. Part (b) shows the survey XPSscan of the AlN capped sample after 1400° C./5 s microwave annealing.Surprisingly, no nitrogen signal can be detected from this scan, but arather strong O 1s signal is seen in addition to the Al signal. Narrowscans (not shown) of the O 1s peak were consistent with the presence ofeither Al₂O₃ or Al(OH)₃.

Thus, upon microwave annealing, the AlN film has oxidized and formedAl₂O₃ and/or Al(OH)₃. This result occurred despite annealing in a99.999% atmosphere of UHP nitrogen, which emphasizes the strongoxidation affinity of the AlN film. A similar result was obtained aftermicrowave annealing at 1500° C./5 s. Part (c) of FIG. 5 shows a surveyXPS scan of the sample after 1400° C. microwave annealing and removal ofthe cap by H₃PO₄. Clearly, Ga and N signals can be observed for this XPSscan, but no Al signals are observed indicating that the AlN cap wassuccessfully removed. Again, a similar XPS scan was obtained for the1500° C. annealed sample, as well as after the AlN cap removal.

3. Surface Morphology of the Microwave Annealed Samples

Tapping mode AFM images may be taken for a) an as-grown GaN surface(RMS=0.3 nm); after 1300° C./5 s microwave annealing of GaN layers with(b) no cap (RMS=9.2 nm), (c) MgO cap (RMS=0.8 nm), and (d) AlN cap(RMS=1 nm); (e) after 1400° C./5 s annealing with MgO cap (RMS=7.2 nm);and (f) after 1500° C./5 s annealing with AlN cap (RMS=0.6 nm). Inshort, these images are based on the GaN sample surface, after microwaveannealing at different temperatures with a MgO cap in place. The MgO capwas able to protect the GaN surface without any substantialdecomposition at annealing temperatures up to 1300° C., but significantGaN decomposition could be detected for the MgO capped annealing done at1400° C. (part (e)). The GaN film totally decomposed, when the microwaveannealing temperature was increased above 1400° C. Decomposition of theGaN was accompanied by cracking of the MgO cap, and liquid Ga dropletscould be observed (not shown) on the surface.

For (d) and (f), microwave annealing took place at 1300° C. and 1500°C., respectively, where both had an AlN cap in place. In (f), the GaNsurface of the 1500° C./5 s microwave annealed sample, with an AlN capin place appears very smooth with a RMS roughness (0.6 nm) comparable tothe as-grown sample (0.3 nm). No evidence of any GaN decomposition canbe seen for even this ultra-high-temperature AlN capped annealing. Forcomparison, (b) refers to an AFM image of a GaN sample annealed for 5 sat 1300° C. without any cap in place. Significant GaN decompositionresulting in the formation of hexagonal cavities can be observed in (b).

To summarize, ultra-fast microwave annealing was successfully used toanneal GaN epilayers up to temperatures as high as 1500° C. using aprotective pulsed laser deposition (PLD) AlN capping layer. The PLDdeposited AlN film is a much better capping layer to preserve thesurface integrity of GaN at temperatures >1300° C. compared to thee-beam deposited MgO film. It might seem that the MgO film might havecracked due to a greater lattice mismatch between the GaN and MgO(˜6.5%) compared to the GaN and AlN (2.6%). However, x-ray diffractionscans (not shown) confirmed that the e-beam deposited MgO layer isfine-grain polycrystalline. Thus, the MgO layer should have plenty ofgrain boundaries to accommodate lattice or thermal co-efficient ofexpansion (TCE) mismatch without cracking. In fact, significant GaNdecomposition was observed for the MgO capped sample annealed at 1350°C., 50° C. before the MgO film cracked, whereas the AlN capped samplesremained decomposition-free even after a 1500° C. treatment. It is knownthat the PLD process used to deposit the AlN cap results in a muchbetter interface with the underlying GaN compared to the e-beamdeposition process, which was used for the MgO cap formation. Thus, thepresence of a large number of voids at the MgO/GaN interface could haveallowed the escape of nitrogen from the GaN film, which accelerated thedecomposition of the GaN film. It would be interesting to explore pulsedlaser deposited MgO caps for protecting the GaN surface duringhigh-temperature microwave annealing.

In addition to the MgO and AlN caps, photoresist converted graphite capswere also explored to study their feasibility for protecting the GaNsurface during high-temperature microwave annealing. Current literaturehas shown that graphite caps have been successfully protected SiCepilayers during ultra-high temperature (1700-1900° C.) microwaveannealing of SiC. However, the present experiments have found that formicrowave annealing of GaN, the graphite caps started delaminating fromthe GaN surface at temperatures >1000° C., presumably because of thestress at the GaN/graphite interface, created by localized decompositionof the GaN epilayer under the graphite cap. From this study, it isevident that an excellent interface between the GaN and the cappinglayer is vital, if the GaN surface morphology is to be preserved duringhigh-temperature annealing.

4. Photoluminescence Characterization

FIG. 6 shows low-temperature (5 K) PL spectra on the in-situ Mg-dopedGaN films annealed at 1300° C. and 1500° C. for a duration of 5 s, usingAlN cap layer. The PL spectrum from an as-grown (unannealed) sample isalso shown for comparison. The only feature visible in the PL spectrafrom the as-grown sample is a broad band (3.0-3.2 eV), with no phononreplicas. This band may be a superposition of at-least two components,the blue luminescence (BL) band and the ultraviolet luminescence (UVL)band. Known to appear at 2.9-3.1 eV in heavily Mg-doped GaN grown byMOCVD, the BL band is supposed to be due to photo-excited carriers froma deep localized donor recombining with the shallow Mg acceptor.Appearing at ˜3.1-˜3.2 eV in heavily Mg-doped and compensated GaN, theUVL band is supposed to originate from the donor-acceptor pair (DAP)recombination between a shallow donor level (presumably O_(N)) and theMg acceptor. The UVL band appears broad and featureless for the as-grownsample. This appearance is possibly due to potential fluctuationsarising from the random distribution of charged impurities such asdonors and acceptors, coupled with the fact that there are not enoughfree carriers to screen them. For the sample annealed at 1300° C., therelative intensity of the blue band reduces. The UVL band increases inintensity and is significantly blue shifted to yield a zero-phonon line(ZPL) at 3.27 eV along with its two LO phonon replicas at 3.18 eV and3.09 eV. Also, a near band-edge emission peak corresponding to therecombination of an exciton bound to a neutral donor (D° X) can also beobserved from FIG. 6. A decrease in the intensity of the blue band andthe appearance of the D° X band indicate that the concentration of thecompensating deep donors has reduced due to the microwave annealing,thus activating the Mg acceptors. This activation can be seen by theblue shift as well as the increase in both intensity and structure ofthe UVL band. For the sample annealed at 1500° C., the relativeintensity of the blue band decreases further, whereas the intensities ofthe UVL band and the near-band-edge emission band increase. Thisindicates that the 1500° C./5 s microwave annealing is more effectivethan the 1300° C./5 s annealing in reducing the concentration of thecompensating deep donor levels and, therefore, in activating the Mgdopants.

Referring to FIG. 7, low-temperature PL spectra, on the microwaveannealed samples with a MgO cap in place, indicate an increase in Mgactivation for 1300° C./5 s annealed sample by the presence of anintense, structured DAP UVL band at 3.29 eV (ZPL) as well as a strongnear-band-edge emission (D° X) band at 3.46 eV. However, upon increasingthe annealing temperature to 1350° C., the D° X band disappears, whereasa broad blue band (2.7-3.1 eV) is observed, which is red shifted evenmore than the 3.0-3.2 eV band observed in the spectra of the as-grownsample. Since the AFM images did show a significant increase in GaNdecomposition for the 1350° C. annealing, it is conceivable that thisbroad band originates from a number of deep donor-like defects (such asV_(N)), which were created by the decomposition. A similar spectra (notshown) was also obtained for the 1400° C./5 s annealed sample, with theMgO cap in place.

The above PL results suggest that high-temperature (1500° C.) microwaveannealing using AlN cap is very effective in increasing the net acceptorconcentration by decreasing the concentration of the compensating deepdonors in GaN.

5. Electrical Characterization

FIG. 8 shows a variation of the hole concentration (p) as a function ofmicrowave annealing temperature, for the uncapped samples and forsamples protected by the MgO and AlN caps during 5 s microwaveannealing. For both uncapped as well as MgO capped samples, the pdecreases, when the annealing temperature is increased above 1300° C.This decrease is a direct result of increasing GaN decomposition withincreasing annealing temperature for uncapped and MgO capped GaN layers,which was observed from the AFM images. The PL spectra for the MgOcapped samples also indicated a decrease in Mg acceptor activation forthe 1350° C. and 1400° C. anneals compared to the 1300° C. annealing,which agrees with the electrical results.

However, for the samples which were capped by the AlN during annealing,the highest p is measured for the 1500° C. annealing. Based on the abovePL results, this phenomenon is likely due to a decrease in thecompensating deep donor concentration with increasing annealingtemperature as long as the integrity of the GaN material is maintained.Relatively high hole mobilities of 14-19 cm2/V.s were measured on allthe above-mentioned samples. No change in the hole mobility after themicrowave annealing treatment was observed.

6. Summary of Microwave Annealing of In-Situ Mg Doped GaN

Because of the ultra-fast heating/cooling rates of the microwave RTAsystem, the GaN can be successfully annealed in the temperature range of1300-1500° C., when the GaN is protected by a pulsed laser deposited AlNcap. The surface of the AlN capped GaN layer annealed at 1500° C. for 5s is very smooth with a RMS roughness of 0.6 nm, which is comparable tothe RMS roughness of 0.3 nm measured on the as-grown sample. The e-beamdeposited MgO cap successfully protected the GaN surface duringmicrowave annealing only up to 1300° C., but a significant GaNdecomposition is observed for the higher temperature anneals.Low-temperature (5 K) PL spectra and Hall measurements performed on theAlN capped samples indicate that the 1500° C./5 s microwave annealing ismore effective than the 1300° C./5 s microwave annealing in activatingthe Mg-dopant by decreasing the concentration of the compensating deepdonor levels present in the as-grown sample. By comparison, fairly goodluminescence and electrical results were obtained for the e-beamdeposited MgO capped GaN layers only for annealing at 1300° C., but theoptical and electrical quality of the GaN layers degrade duringhigher-temperature (>1300° C.) annealing. Photoresist converted graphitecap delaminates from the GaN surface for microwave annealingtemperatures >1000° C. and is therefore not a suitable capping materialfor high-temperature annealing of GaN.

C. Microwave Annealing of Mg-Implanted GaN

After demonstrating improvement in the optical and electrical propertiesof in-situ Mg-doped GaN after high-temperature (1300-1500° C.) microwaveannealing, the logical next step was to explore the feasibility ofmicrowave annealing on Mg ion-implanted GaN. The Mg-implanted GaN layerscould be used as the base region in a GaN heterojunction bipolartransistor (HBT). Also, selective Mg-implants through an implantationmask could be used to more easily create arrays of GaN LEDs and laserdiodes as opposed to reactive ion etching p-type GaN epilayers.

1. EXAMPLE 1

a. Implantation and Annealing Schedules

TABLE 3 shows the multiple energy Mg⁺ implant schedule performed intoundoped 3 μm GaN epilayers grown on a-plane sapphire.

TABLE 3 Multiple Energy Mg Implant Schedule Performed into Undoped GaNImplant Energy (keV) Implant Dose (cm⁻²) 10 3.8 × 10¹³ 25 3.3 × 10¹⁴ 551.7 × 10¹⁴ 110 4.1 × 10¹⁴ 225 8.3 × 10¹⁴ 300 8.3 × 10¹⁴

The implantation was performed at a temperature of 500° C. with a tiltof 7°. As in case of SiC, the multiple energy Mg implant schedule forGaN was also designed using the SRIM-2006 software. FIG. 9 shows acomparison of the simulated and the experimental (SIMS) Mg implantprofiles. It can be observed that there is a significant discrepancybetween the simulated and the experimentally determined Mg implantprofiles. The simulation predicts a higher Mg concentration and asmaller ion penetration depth, whereas the experimentally measuredprofile displays a longer implant tail into the substrate. A similardiscrepancy between simulated and experimental Si implant profiles inGaN was observed in previous literature. Thus, some work will need to bedone to obtain better stopping powers for implanted ions in GaN.

After implantation, the GaN epilayers were capped by a 0.3 μm layer ofAlN grown by PLD and then subjected to microwave annealing in the rangeof 1300-1500° C. After annealing, the AlN cap was etched by the H₃PO₄recipe, as described earlier. The reliable removal of the AlN caps aftermicrowave annealing was again confirmed by the XPS measurements. Afterremoving the cap, the Mg-implanted GaN epilayers were characterized fortheir structural and electrical properties, and also for the thermalstability of the implant.

b. SIMS Depth Profiling

FIG. 10 shows SIMS depth profiles of the Mg implanted GaN before andafter 1300° C./5 s and 1400° C./5 s microwave annealing. The SIMSprofile for the as-implanted sample and for the 1300° C./5 s annealedsample are close. However, a slight Mg accumulation at the surface andsome in-diffusion of Mg into the GaN can be observed for the 1300° C.annealed sample. The microwave annealing at 1400° C. resulted in asignificant Mg accumulation in a thin ≈40 nm surface layer, and adepletion of Mg at depths of 40 nm-400 nm from the surface. A pronouncedin-diffusion of Mg into the GaN can also be observed at depths beyond400 nm. As illustrated, the extracted doses from the 1300° C. (1.6×10¹⁵cm⁻²) and 1400° C. (1.5×10¹⁵ cm⁻²) annealed samples are slightly lowercompared to the extracted dose (1.7×10¹⁵ cm⁻²) from the as-implantedsample. This difference is probably due to some out-diffusion of Mg intothe AlN cap during the annealing treatment.

c. Photoluminescence Characterization

FIG. 11 shows low-temperature PL spectra from Mg-implanted GaN, beforeand after 1400° C./5 s and 1500° C./5 s microwave annealing. Forreference, the PL spectra from an as-grown GaN epilayer used for theMg-implantation is also shown. In addition to the near-band edgeemission, a broad yellow luminescence (YL) band (2.0 eV-2.6 eV) and abroad blue luminescence (BL) band (2.7 eV-3.2 eV) also can be observedin the PL spectra obtained from the as-grown GaN epilayer. As discussedearlier, the appearance of BL in low-temperature PL spectra of GaN isattributed to the presence of donor-like states in the bandgap, whichmay arise from V_(N) or pyramidal defects. The YL in GaN is generallyattributed to the presence of C, O, and H in the material. The presenceof YL and BL in the PL spectra indicates a poor quality GaN material,especially for p-type doping, since the YL and BL can severelycompensate the activated acceptors.

The as-implanted GaN does not exhibit any photoluminescence, since theimplant-induced damage introduces a lot of defect levels in the bandgap,which act as non-radiative recombination centers. The PL spectra fromthe 1400° C. microwave annealed GaN does show the re-appearance of thenear band-edge D° X emission as well as DAP emission related to Mgactivation. Thus, microwave annealing at 1400° C. at-least partiallyheals the implant-induced lattice damage. Increasing the annealingtemperature to 1500° C. results in further recovery of implant-induceddamage as can be seen from the increase in intensity of both D° Xemission and Mg activation related DAP emission from the PL spectra ofFIG. 11. However, the YL and BL bands can also be seen in the PL spectraof microwave annealed samples, which possibly precludes any electricalactivation due to compensation of the activated acceptors. Thus, it isparamount to have an excellent quality GaN epilayer which doesn't emitYL and BL, especially for fabricating device structures which requirep-type implantation.

d. Electrical Characterization

Electrical characterization of the GaN even after a 1500° C. annealingtreatment has indicated almost no electrical activation of Mg. Thesamples are highly resistive. This result is likely due to thesignificant lattice damage created by the high dose, multiple energy Mgimplant. Also, as shown in FIG. 11, the PL spectra have indicated thepresence of compensating deep levels even in the as-grown GaN epilayer.

2. EXAMPLE 2

a. Experimental Details

Single-energy (150 KeV) Mg implantation was performed intounintentionally n-type doped, 3 μm thick GaN films grown on c-planesapphire using hydride vapor phase epitaxy (HVPE). An implantation doseof 5×10¹⁴ cm⁻² was applied on the wafer tilted 7° and kept at 500° C.Multiple-energy Mg ion-implantation was also performed at 500° C. insome samples, using the schedule shown in TABLE 3. The implanted surfaceof the GaN samples was then capped with 6000 Å thick AlN layer using PLDto protect the surface of the GaN films during the high temperatureannealing process.

Microwave annealing was performed on implanted GaN films using a highlyconducting 4H—SiC piece placed directly underneath the sample to serveas a susceptor. During the annealing, the implanted surface ofGaN/Sapphire, capped with thin AlN film, was placed with the GaN facingdown on a polished surface of the 4H—SiC susceptor. It is important tomention that the SiC susceptor is not required if the GaN layer is grownon a conducting hexagonal SiC substrate. However, for extra protectionof GaN surface, it is advisable to place the AlN capped GaN layer inintimate contact with another polished SiC or sapphire piece. Both thesample and the susceptor were placed in the microwave heating head andthe temperature was measured using an optical pyrometer. Microwaveannealing was performed in a temperature range of 1300-1600° C. for 5-15s in pure (99.9%) nitrogen atmosphere. After annealing, the AlN cap wasremoved by a 1 hour etch in 85% wt H₃PO₄ at 120° C. The removal of theAlN cap from the GaN surface was confirmed by XPS.

The sample surface is monitored, after annealing and removal of the cap,by tapping mode AFM. High-resolution X-ray diffraction measurements weretaken using an 18 kW Rigaku ATX-E diffractometer with Cu radiation. Twochannel cut Ge (220) crystals were used to monochromatize the incidentbeam and provide parallel beam of CuKα₁ radiation. This arrangementalmost eliminates the vertical divergence in the incident beam andprovides a precise measurement of the rocking curve and latticeparameters. In addition, this diffractometer is equipped with an openEulerian cradle with independent (x,y,z) movements and the tilt, χ androtation, φ movements which are necessary for precise alignment of thesample. The optical characterization of the as-grown, implanted, andannealed samples was performed using low-temperature (5 K)photoluminescence spectroscopy. The luminescence was excited with the325 nm line of a He—Cd laser with an excitation intensity of 2.5 mW. Thelight emiited by the samples was dispersed by a high-resolutionspectrometer fiited with a UV-sensitive photomultiplier coupled to aphoton counter. SIMS measurements were also performed on the as-dopedand annealed samples to check the thermal stability of the dopants.

b. AFM and SIMS Configuration

Tapping mode AFM images may be taken of single energy Mg-implanted GaNsample surface, after 1300° C./15 s, and 1500° C./15 s microwaveannealing. The rms roughness before annealing is 0.3 nm. After 1300°C./15 s annealing, it is 1.5 nm. The rms surface roughness increasedwith increasing annealing temperature. The AlN cap provided a reasonablesurface protection for GaN even for 1500° C./15 s annealing, yielding anrms roughness of 6 nm. The results shown are for the samples in whichthe AlN capped (600 nm thick) implanted surface was placed face down onthe polished surface of the 6H—SiC susceptor. For samples with smallerAlN cap thickness and where the AlN capped surface is not placed inintimate contact with the SiC susceptor, an rms surface roughness of 50nm or more was observed after 1500° C./15 s annealing. Based on theseresults, even if the GaN film is grown on a conductive hexagonal SiCsubstrate, it is advisable to place it face down on a polished SiCsubstrate to preserve the surface morphology.

Referring to FIG. 10 again, Mg implant SIMS depth profiles may berecorded on as-implanted and microwave annealed (1300° C./5 s and 1400°C./5 s) multiple energy Mg-implanted samples. The SIMS profile for theas-implanted sample and for the 1300° C./5 s annealed sample aresimilar. However, a slight Mg accumulation at the surface and somein-diffusion of Mg into the GaN film (toward the GaN-substrateinterface) can be observed for the 1300° C. annealed sample. Themicrowave annealing at 1400° C. resulted in a significant Mgaccumulation in a thin ≈40 nm surface layer, and a depletion of Mg atdepths of 40 nm-400 nm from the surface. A pronounced in-diffusion of Mginto the GaN can also be observed at depths beyond 400 nm. The extracteddoses from the 1300° C. (1.6×10¹⁵ cm⁻²) and 1400° C. (1.5×10¹⁵ cm⁻²)annealed samples are slightly lower compared to the extracted dose(1.7×10¹⁵ cm⁻²) of the as-implanted sample. This finding may result fromsome out-diffusion of Mg into the AlN cap during the annealingtreatment.

c. X-Ray Characterization

FIG. 12 shows HRXRD spectra of the GaN (004) reflections of theas-implanted and microwave annealed multiple energy Mg-implantedsamples. It can be seen from the HRXRD spectra that the sub-latticedefect peak, which is due to the interference of the X-rays from theimplanted impurity in the interstitial sites, did not disappear afterannealing. This peak results from the excessive damage produced by themultiple energy implantation process in the GaN films, which could notbe eradicated by the microwave annealing process. The lattice parametersand rocking curve full width at half maximum (FWHM) values for themultiple-energy Mg-implanted GaN films show no significant improvementupon annealing of these samples. Because the multiple-energyimplantation process caused severe damage in the samples, single-energyMg ion-implantation was performed for further study of Mg-acceptoractivation in GaN.

FIG. 13 shows an overlay of the XRD scans of the GaN (004) reflectionsof the as-grown, as-implanted, and microwave annealed single-energyMg-implanted GaN samples. It can be seen that the implanted sample hasthe defect sub-lattice peak as shown earlier in FIG. 12 for the multipleenergy Mg-implantation. For single-energy Mg-implantation, the defectsub-lattice peak disappears after microwave annealing (unlike inmultiple energy Mg-implanted samples), confirming that the implanted Mghas moved to electrically and optically active substitutional latticepositions. In addition, the FWHM values, shown in TABLE 4, also decreaseupon annealing, indicating that the implant induced damage has beenremoved. Upon 1300° C./15 s annealing, the FWHM value decreases to theun-implanted sample level. For higher temperature anneals, the FWHMvalue goes down further and reaches towards values lower than that ofthe un-implanted samples. Consistent with previously reported resultsfor ion-implanted SiC, this observation suggests that the hightemperature microwave annealing process may also remove some of thegrowth related defects in the un-implanted region of GaN films,improving their crystalline quality. This effect is possible, becausethe GaN films growth temperatures are below the microwave annealingtemperatures.

TABLE 4 a and c Lattice Parameters and Rocking Curve FWHM Values of theVirgin, As-implanted, and 15 s Microwave Annealed Single-EnergyMg-implanted GaN Films FWHM C, A Sample Reflection Θ (arcsecs) 2Θ (Å)Virgin (004) 36.745 293 72.900 5.1860 (104) 16.918 209 82.040 3.1890 asimplanted (004) 36.819 310 72.900 5.1860 (104) 16.741 231 82.027 3.19151300° C. (004) 36.132 273 72.908 5.1855 (104) 15.345 197 82.036 3.19141400° C. (004) 36.098 248 72.907 5.1856 (104) 15.508 206 82.035 3.19131500° C. (004) 36.732 252 72.906 5.1857 (104) 16.879 207 82.038 3.1906

d. PL Measurements

FIG. 14 shows low-temperature PL spectra from multiple-energyMg-implanted GaN samples, before and after 1400° C./5 s and 1500° C./5 smicrowave annealing. Also seen in the PL spectra obtained from theas-grown GaN epilayer is the near-band edge (NBE) emission, a broadyellow luminescence (YL) band (2.0 eV-2.6 eV), and a near-ultra violetluminescence (N-UVL) band (2.7 eV-3.2 eV). The YL band in GaN isgenerally associated to the presence of C, O, and/or H in the material.The YL and N-UVL bands in the PL spectra indicate the presence of deeplevels in un-intentionally doped (UID) as-grown GaN films.

The as-implanted GaN sample does not exhibit any significantphotoluminescence in the probed spectral range. Implantation-induceddamage introduces a number of defects in the bandgap. Such defects actas non-radiative recombination centers or radiative centers emitting indifferent spectral range. The PL spectrum of the 1400° C. microwaveannealed sample show a weak NBE emission, N-UVL bands (2.7 eV-3.2 eV),and a relatively intense YL band. Thus, microwave annealing at 1400° C.at least partially heals the implantation-induced lattice damage. The PLspectrum of the 1500° C. annealed sample shows a relatively largeincrease of the NBE and N-UVL band intensities. Meanwhile, no intensityvariation is observed for the YL band. In addition, it can be clearlyobserved that at ˜3.26 eV, the no-phonon line of the recombinationprocess associated with shallow DAP. Therefore, increasing the annealingtemperature to 1500° C. results in further reduction of theimplantion-induced damage. However, the defects related to the YL andN-UVL bands are still the dominant recombination channels as comparedwith those involving NBE and DAP emission bands, which are consistentwith the small activation of the Mg-impurities and persistent latticedamage.

FIG. 15 shows low-temperature PL spectra of the single-energyMg-implanted GaN annealed at 1300° C., 1400° C., 1500° C. (for 15 s),and the as-implanted (un-annealed) samples. Similar to the PL spectrumof the un-annealed multiple-energy Mg-implantation shown in FIG. 14, thespectrum of the un-annealed single-energy Mg-implantation, representedin FIG. 15, shows an overall decrease of the luminescence intensity.However, the NBE luminescence emission of the latter did not evanescelikewise that of the former. This result is consistent with the XRDresults, indicating lower ion-implantation lattice damage. Similar tothe multi-energy implanted sample set, the intensity of the YL reach itsmaximum at 1300° C./15 s, showing no strong dependence with theannealing temperature, while the N-UVL intensity increases withincreasing annealing temperature. The no-phonon line of the DAP band isclearly observed in the spectra of the samples annealed at 1400° C. and1500° C., overlapping the N-UVL band. It is difficult to identify thephonon replicas of the DAP due to the interference fringes modulatingthe spectra. An emission band between 3.27 eV and 3.37 eV is observed inthe spectra of the sample annealed at 1300° C., and may be weaklyobserved in the 1400° C. annealed sample spectrum. But, it is completelyquenched in the 1500° C. annealed sample spectrum. This band may beassociated with a structural defect with low thermal annealingtemperature. Additional experimentation is necessary to determine itsnature. It should be noted that despite the increase of the N-UVL andDAP band intensities, the intensity of the NBE emission band did notincrease. This lack of increase may be consistent with increasingconcentration of the Mg-acceptor, along with increasing annealingtemperature, which compensate the shallow donor(s) associated with theNBE emission band.

e. Electrical Configuration

The electrical characterization of the multiple energy Mg-implanted GaNfilms, even after a 1500° C. annealing treatment, has indicated almostno net electrical activation of the Mg-acceptor implant. The samplesremained highly resistive after annealing and are likely due to the highdegree of lattice damage created by the high-dose, multiple-energyMg-implantation. Though the small DAP related band was observed in thePL spectra of the annealed samples, the x-ray measurements indicated ahigh degree of residual implant damage in the annealed samples. Also,the XRD spectra (not shown) and PL spectra (FIG. 14) indicated thepresence of defects even in the as-grown GaN films used for the multipleenergy implantation, which may influence the Mg activation. Thecombination of the poor starting material and high residual implantdamage remaining in the material even after 1500° C. annealing aredetrimental for the electrical activation of the multiple-energy Mgimplant. No electrical activation was observed even after 15 s annealingat 1500° C.

FIG. 16 shows two-probe current-voltage (I-V) measurements on thesingle-energy Mg implanted GaN films and acceptor activation in themicrowave annealed samples when the samples were annealed for 15 s. Nonet electrical activation was observed for 5 s annealing. The breakdownvoltage decreased with increasing annealing temperature, for 15 sannealing, indicating an increase in the Mg-acceptor activation withincreasing annealing temperature. The extrapolated resistivity from thelinear segments of the I-V curves is in the range of 0.4 to 1.4 Ω-cm forannealing temperatures of 1500° C. to 1300° C. Hence, based on the XRD,PL and I-V measurements it can be stated that the single energy Mgimplanted films, after 15 s high temperature microwave annealing haveshown improved crystalline quality and activation of the Mg-implant. Thenet Mg acceptor activation stems from an increase in Mg substitutionalacceptor activation coupled with an effective decrease in the density ofcompensating background donor defects (like residual nitrogen vacancyintroduced by implantation).

D. Summary and Future Work on Microwave Annealing of GaN

For Example 1, GaN epilayers were reliably annealed at high-temperaturesin the range of 1300-1500° C., when the GaN is protected by a PLD AlNcap. Promising electrical and optical results were obtained for in-situMg doped epilayers. However, it has proven to be more challenging toactivate a multiple energy, high dose Mg implanted GaN. Significantlattice damage exists even after annealing at temperatures as high as1500° C., albeit for short 5 s durations. Lower dose single-energy Mgimplantations are planned on GaN epilayers, which are of a much higherquality than the ones explored in the present study.

For Example 2, a novel ultra-fast microwave annealing method, differentfrom conventional thermal annealing, is used to successfully activateMg-implants in GaN layer. The x-ray diffraction measurements indicatedcomplete disappearance of the defect sub-lattice peak, introduced by theimplantation process for single-energy Mg-implantation, when theannealing was performed at ≧1400° C. for 15 s. An increase of theintensity of Mg-acceptor related luminescence peak (at 3.26 eV) in thephotoluminescence spectra combined with a decreasing Schottky breakdownvoltage confirm the net Mg-acceptor activation of single-energyMg-implanted GaN. In the case of multiple-energy implantation, theimplant generated defects persisted even after 1500° C./15 s annealing,resulting in no net Mg-acceptor activation of the Mg-implant. TheMg-implant is relatively thermally stable and the sample surfaceroughness is 6 nm after 1500° C./15 s annealing, using a 600 nm thickAlN cap.

It is found that p-type activation can be achieved by microwaveannealing of single-energy Mg-implanted GaN for annealing temperaturesequal to or above 1400° C. for 15 s. The XRD and PL results and thetwo-probe current-voltage measurements confirm the effective removal oflattice damage and Mg acceptor activation in single-energy Mg-implantedGaN films. For multiple-energy Mg-implantation, we could not observe netp-type conduction even for 15 s annealing due to a large degree ofimplant lattice damage.

Future work involves ultra-high temperature annealing of Si (n-type)implanted GaN and especially AlGaN epilayers grown on SiC. Ifsuccessful, such layers can be used under source/drain metal contacts ofAlGaN—GaN HEMT devices, in an attempt to lower the source/drain accessresistance, and to increase the device transconductance. Also, futurehigh-temperature microwave anneals are planned on in-situ Mg dopedAl_(0.25)Ga_(0.75)N and Al_(0.4)Ga_(0.6)N grown on sapphire. Increasingthe Al content in the AlGaN ternary increases the bandgap and findsapplication in smaller wavelength laser diodes. However, the increasingAl content in AlGaN also makes p-type doping more difficult to achieve.

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VI. Statements

While various embodiments have been described above, it should beunderstood that they have been presented by way of example, and notlimitation. It will be apparent to persons skilled in the relevantart(s) that various changes in form and detail can be made thereinwithout departing from the spirit and scope. In fact, after reading theabove description, it will be apparent to one skilled in the relevantart(s) how to implement alternative embodiments. Thus, the presentembodiments should not be limited by any of the above describedexemplary embodiments.

In addition, it should be understood that any figures which highlightthe functionality and advantages, are presented for example purposesonly. The disclosed architecture is sufficiently flexible andconfigurable, such that it may be utilized in ways other than thatshown. For example, the steps listed in any flowchart may be re-orderedor only optionally used in some embodiments.

Further, the purpose of the Abstract of the Disclosure is to enable theU.S. Patent and Trademark Office and the public generally, andespecially the scientists, engineers and practitioners in the art whoare not familiar with patent or legal terms or phraseology, to determinequickly from a cursory inspection the nature and essence of thetechnical disclosure of the application. The Abstract of the Disclosureis not intended to be limiting as to the scope in any way.

Finally, it is the applicant's intent that only claims that include theexpress language “means for” or “step for” be interpreted under 35U.S.C. 112, paragraph 6. Claims that do not expressly include the phrase“means for” or “step for” are not to be interpreted under 35 U.S.C. 112,paragraph 6.

1. A doped gallium nitride (GaN) annealing method comprising: a.placing, within a heating unit, a silicon carbide (SiC) wafer as asusceptor in close proximity with a doped GaN epilayer, wherein thedoped GaN epilayer is either a GaN layer grown on a substrate or a GaNlayer that is free standing; and b. heating, at a heating rate of atleast about 100° C./s, the wafer and the doped GaN epilayer to at leastabout 1200° C.
 2. The method according to claim 1, further includingdepositing a protective capping layer on the doped GaN epilayer.
 3. Themethod according to claim 2, wherein the protective capping layer isaluminum nitride (AlN).
 4. The method according to claim 3, wherein theAlN is deposited using pulsed-laser deposition.
 5. The method accordingto claim 2, wherein the protective capping layer is magnesium oxide(MgO).
 6. The method according to claim 1, wherein the method isperformed in a vacuum chamber or an inert gas atmosphere.
 7. The methodaccording to claim 1, wherein the doped GaN is either in situ doped orion-implantation doped.
 8. The method according to claim 1, wherein adopant used for the doped GaN is: magnesium, beryllium, calcium, zinc,silicon, sulfur, iron, cobalt, vanadium, or any combination thereof. 9.The method according to claim 1, wherein the heating unit is a microwaveheating head.
 10. The method according to claim 1, wherein the heatingunit is a laser annealing system.
 11. A doped gallium nitride (GaN)annealing method comprising: a. placing, within a heating unit, a dopedGaN epilayer, the doped GaN epilayer being either a GaN layer grown on aconducting substrate or a GaN layer that is free standing; and b.heating, at a heating rate of at least about 100° C./s, the doped GaNepilayer to at least about 1200° C.
 12. The method according to claim11, further including depositing a protective capping layer on the dopedGaN epilayer.
 13. The method according to claim 12, wherein theprotective capping layer is aluminum nitride (AlN).
 14. The methodaccording to claim 13, wherein the AlN is deposited using pulsed-laserdeposition.
 15. The method according to claim 12, wherein the protectivecapping layer is magnesium oxide.
 16. The method according to claim 11,wherein the doped GaN is either in situ doped or ion-implantation doped.17. The method according to claim 11, wherein a dopant used for thedoped GaN is: magnesium, beryllium, calcium, zinc, silicon, sulfur,iron, cobalt, vanadium, or any combination thereof.
 18. The methodaccording to claim 11, wherein the method is performed in a vacuumchamber or an inert gas atmosphere.
 19. The method according to claim11, wherein the heating unit is a microwave heating head.
 20. The methodaccording to claim 11, wherein the heating unit is a laser annealingsystem.